Whilst the plugin tries to work on the most common PHP environments, it
currently requires PHP 5.4 or higher.

As official security support drops for PHP 5.6 at the end of 2018, it is
strongly recommended to upgraded your WordPress hosting environment to
PHP 7, ideally, PHP 7.1 or 7.2, as 7.0 will also stop being supported in
December, 2018.